BOSTON -- Gov. Charlie Baker, a strong advocate for charter schools, is using his fiscal year 2017 budget proposal to change the way in which district schools are reimbursed for charter school tuition.

Baker wants to give district schools an extra $20.5 million next year through the charter school reimbursement formula, for a total of $101 million, with most of the extra money going to poorly performing school districts that have large numbers of charter school students. But the formula would give most districts less money in future years than they are entitled to under the current law.

The change comes as lawmakers are debating whether to lift a cap on charter schools, a move Baker supports and the state's teachers' unions oppose. Pro-charter school activists are working to put a question on lifting the cap on the 2016 ballot.

One argument against lifting the cap is that district schools lose money when students switch to charter schools.

"Folks who expressed this concern should feel some comfort that we're trying to take action and ensure that districts faced with that transition have the resources they need in order to get through it without having to reduce services or infringe on quality," Education Secretary Jim Peyser said in an interview.

Baker said the extra money shows that his administration is willing to reform the funding formula as part of discussions over lifting the cap.

"Our proposal with respect to the charter school funding formula is an attempt on our part to acknowledge this is a debate and a discussion that needs to be had, and we look forward to engaging with the Legislature on this," Baker said. "It is certainly our hope we can use this as an opportunity to do what many families and their children would like to see us do, which is lift the charter cap."

But teachers' unions say that in the long term, the proposed funding formula would hurt them.

"I think it is more evidence that our governor is not committed to fully funding public education," said Barbara Madeloni, president of the Massachusetts Teachers Association.

The funding formula is complex. The basic idea is when a student attends a charter school, the money the state would pay to educate that student is deducted from the amount the state pays the district and instead goes to the charter school. The district is then reimbursed for a portion of that money. The district today is supposed to get 100 percent of the cost the first year after a student leaves and 25 percent of the cost for the next five years. The point is to help school districts adjust to having fewer students.

However, the state has traditionally not had money to fully fund the reimbursement formula. So for the last two years, the state has only paid part of the first year of reimbursements and paid nothing for the next five years.

Baker's proposal would change the formula to only provide reimbursement for three years, rather than six years. The formula would give a 100 percent reimbursement the first year, 50 percent the second year and 25 percent the third year. However, the second and third year funding would be reserved for districts that are low-performing and have high numbers of charter school students. These are districts that today are spending more than 9 percent of their school budgets on charter tuition payments.

There are nine districts that fall into this category, including Holyoke and Springfield. More than half of charter school students statewide are in these districts.

In fiscal year 2017, Baker's proposed budget would pay for the first two years of the funding formula. Peyser said the administration intends to fund all three years of the formula the following year.

The budget also says that in order to receive the second and third years of funding, a district must submit a plan that shows how it will use the money to maintain and improve the quality of services.

Baker's proposal is similar, though not identical, to a plan previously proposed by Boston Mayor Marty Walsh.

Marc Kenen, executive director of the Massachusetts Charter Public School Association, praised the proposal. Kenan said he hopes the additional money for districts can encourage lawmakers to lift the cap, since the money would offset the impact of creating new charter schools. "We're very encouraged that the governor has engaged the Legislature on the charter district reimbursement program, and we're hopeful that might break the logjam on the cap lift in the Senate," Kenen said.

Eileen O'Connor, a spokeswoman for Great Schools Massachusetts, a coalition of charter school supporters working to lift the cap, said, "We support the Governor's budget proposal, and urge legislators to take it to heart as they consider legislation that would lift the cap and give 37,000 kids access to great public schools in Massachusetts."

But Madeloni countered that the formula makes lifting the cap less appealing, since it means that districts - particularly those that do not qualify for second and third year funding - would be eligible for less money.

"I don't even know what to say about a governor who says 'Jeez, we haven't been giving them enough money to begin with, so it doesn't matter if we're not going to continue to have enough money,'" Madeloni said.

Madeloni said the formula will destabilize districts that are only eligible for one year of funding.

"If the governor was interested in high quality public education for every child in Massachusetts... he would be talking about fully funding every public school not bleeding more funds from public education," Madeloni said.

The state Senate is currently trying to develop its own plan for reforming the charter school law and lifting the cap. Asked whether the Senate might go along with Baker's proposal, a spokesman for Senate President Stan Rosenberg, D-Amherst, said Rosenberg is still reviewing it.